In an oil and gas well, cement between the casing and a borehole is designed to provide zonal isolation in the wellbore. However, liquid contaminants or defect created by volumes of inadequate density might lead to failed isolation. Fixing these defects can be expensive and difficult. Methods and devices for detecting defects in the cement and analyzing the quality of the cement are important to the oil and gas industry. One technique common used is acoustic logging. Acoustic logging detects and assesses the cement behind casing by measuring the acoustic impedance of the cement bonded to the casing. However, conventional acoustic tools are subject to important limitations. For example, acoustic decoupling can be caused by the presence of a microannulus or a shear film coating in the casing. A microannulus is typically a small gap that forms between the casing the surrounding cement. A microannulus may allow the casing to move, thus breaking the bond between the cement and the casing. A microannulus may be partial, or in some instances, it may extend around the entire casing circumference. This may allow undesirable fluid communication between zones. Even newly developed ultrasonic imaging tools, which are based on pulse-echo techniques, are limited when heavy mud or thick casing is used, since the reflected signals die away very quickly. For example, the typical upper limit of the casing thickness allowable in pulse-echo ultrasonic measurement is about 0.59 inch. Therefore, this technique is typically only suitable if the volume of interest is very close to the casing and strongly bonded to the reflective surfaces. Otherwise, such defects may go undetected.
Accordingly, a need exists for an improved downhole imaging method and devices.